1. Field of the Invention
The present invention relates to a method of evaluating a deteriorated state of an exhaust gas adsorbent for adsorbing an exhaust gas emitted from an internal combustion engine.
2. Description of the Related Art
Exhaust gas purifying systems for internal combustion engines generally have a three-way catalytic converter disposed in the exhaust passage of an internal combustion engine for removing unburned components including HC (hydrocarbon), NOx (nitrogen oxides), and CO (carbon oxide) from the exhaust gas emitted from the internal combustion engine.
Generally, the catalytic converters are not activated unless their temperature reaches a certain level, and do not perform an intended exhaust gas purifying function while they are being inactivated at low temperatures. Therefore, the exhaust gas purifying systems for internal combustion engines are required to increase its exhaust gas purifying capability at temperatures where the exhaust gas purifying function of the catalytic converter is not fully performed, e.g., immediately after the internal combustion engine has started up when the internal combustion engine and the catalytic converter are cold.
To meet the above requirement, there has been proposed an exhaust gas purifying systems having a catalytic converter and also an exhaust gas adsorbent for adsorbing a particular gas component, e.g., HC, of the exhaust gas, disposed in the exhaust passage of an internal combustion engine, as disclosed in Japanese laid-open patent publications Nos. 8-93458 and 8-218850, for example.
In the proposed exhaust gas purifying system, an exhaust passage downstream of the catalytic converter is divided into two exhaust passages, one of which (hereinafter referred to as xe2x80x9cauxiliary exhaust passagexe2x80x9d) houses the exhaust gas adsorbent for adsorbing HC therein. The auxiliary exhaust passage with the auxiliary exhaust passage housed therein is joined to the other exhaust passage (hereinafter referred to as xe2x80x9cmain exhaust passagexe2x80x9d) downstream of the exhaust gas adsorbent. The joint between the auxiliary and main exhaust passages is associated with a switching valve which selectively vents one of the auxiliary and main exhaust passages to the atmosphere and closes the other exhaust passage from the atmosphere. The auxiliary exhaust passage is connected to an exhaust passage upstream of the catalytic converter via a recirculation path with an on/off valve from a location downstream of the exhaust gas adsorbent and upstream of the switching valve.
The exhaust gas adsorbent comprises zeolite or the like, and has such characteristics that it adsorbs HC when its temperature is relatively low and desorbs the adsorbed HC when its temperature rises.
The exhaust gas purifying system operates as follows: In a period (hereinafter referred to as xe2x80x9cabsorption periodxe2x80x9d) after the internal combustion engine has started up until the temperature of the exhaust gas adsorbent and the temperature of the exhaust gas or the time elapsed after the internal combustion engine has started up exceeds a predetermined value, the on/off valve in the recirculation path is closed and the auxiliary exhaust passage is vented to the atmosphere and the main exhaust passage is shut off from the atmosphere by the switching valve, so that the exhaust gas emitted from the internal combustion engine is discharged through the catalytic converter and the exhaust gas adsorbent, i.e., HC adsorbent, into the atmosphere. Immediately after the internal combustion engine has started up, the catalytic converter is unable to sufficiently remove HC from the exhaust gas because the catalytic converter is cold. However, since the exhaust gas adsorbent that adsorbs HC at a relatively low temperature is also cold, while the exhaust gas having passed through the catalytic converter is passing through the auxiliary exhaust passage, the exhaust gas adsorbent absorbs HC in the exhaust gas and hence removes HC from the exhaust gas.
In a subsequent period (hereinafter referred to as xe2x80x9cdesorption periodxe2x80x9d) after the adsorption period is ended until the temperature of the exhaust gas adsorbent and the temperature of the exhaust gas or the time elapsed after the internal combustion engine has started up exceeds a predetermined value, the main exhaust passage is vented to the atmosphere and the auxiliary exhaust passage is shut off from the atmosphere by the switching valve, and the on/off valve in the recirculation path is opened, so that a portion of the exhaust gas emitted from the internal combustion engine is recirculated via the auxiliary exhaust passage and the recirculation path to the exhaust passage upstream of the catalytic converter, and the exhaust gas is discharged through the catalytic converter into the atmosphere. As the exhaust gas supplied to the auxiliary exhaust passage is recirculated to the exhaust passage upstream of the catalytic converter, the HC adsorbed by the exhaust gas adsorbent in the adsorption period is desorbed from the exhaust gas adsorbent and supplied to the catalytic converter. The HC supplied to the catalytic converter is removed by the catalytic converter which has been increased in temperature and activated in the adsorption period.
The exhaust gas purifying system is thus capable of purifying the exhaust gas while the catalytic converter is being inactivated.
The exhaust gas adsorbent for adsorbing a particular gas component, e.g., HC, of the exhaust gas is deteriorated due to aging, and will not be able to sufficiently adsorb the particular gas component if it is deteriorated to a certain degree. The exhaust gas adsorbent thus deteriorated is unable to perform the desired exhaust gas purifying capability. It is therefore desirable evaluate a deteriorated state of the exhaust gas adsorbent according to some process so that the exhaust gas adsorbent can be replaced when it is deteriorated to some extent.
One proposal for evaluating a deteriorated state of the exhaust gas adsorbent is disclosed in Japanese laid-open patent publication No. 8-218850, for example.
According to the disclosed proposal, exhaust gas sensors for generating an output depending on the concentration of HC adsorbed by the exhaust gas adsorbent, e.g., an O2 sensor (oxygen concentration sensor), an air-fuel ratio sensor, an HC sensor, etc., are disposed upstream and downstream of the exhaust gas adsorbent. A time required until the output from the downstream exhaust gas sensor becomes equal to the output from the upstream exhaust gas sensor is measured in the desorption period after the adsorption period. While the HC is being desorbed from the exhaust gas adsorbent, since the HC concentration downstream of the exhaust gas adsorbent is higher than the HC concentration upstream of the exhaust gas adsorbent, the output from the downstream exhaust gas sensor is greater than the output from the upstream exhaust gas sensor.
If the measured time does not fall in a predetermined range, then it is determined that the exhaust gas adsorbing system suffers a failure, e.g., the exhaust gas adsorbent is deteriorated, the exhaust gas leaks from the switching valve, etc.
In order to evaluate a deteriorated state of the exhaust gas adsorbent, however, the above proposed process is based on the assumption that the time measured in the desorption period remains substantially constant insofar as the exhaust gas adsorbent is not deteriorated. The assumption means that insofar as the exhaust gas adsorbent is not deteriorated, the total amount of HC adsorbed by the exhaust gas adsorbent in the adsorption period remains substantially the same, and the absorbed HC is desorbed from the exhaust gas adsorbent at substantially the same rate in the desorption period.
The amount of HC adsorbed by the exhaust gas adsorbent in the adsorption period and the rate at which the absorbed HC is desorbed from the exhaust gas adsorbent in the desorption period are affected by the rate of the exhaust gas supplied to the auxiliary exhaust passage with the auxiliary exhaust passage housed therein. According to the process disclosed in Japanese laid-open patent publication No. 8-218850, the above assumptions may not often be true depending on the operating state of the internal combustion engine immediately after it has started up. The deteriorated state of the exhaust gas adsorbent cannot properly be evaluated if the above assumptions are no longer applicable.
In Japanese laid-open patent publication No. 8-218850, the exhaust gas sensors, e.g., an O2 sensor, an air-fuel ratio sensor, an HC sensor, etc., which are disposed upstream and downstream of the exhaust gas adsorbent, are sensitive to various exhaust gas components, e.g., CO, H2, O2, etc., other than HC adsorbed by the exhaust gas adsorbent. According to the process disclosed in Japanese laid-open patent publication No. 8-218850, if the exhaust gas components other than HC in the exhaust gas varies, then it is frequently impossible to properly obtain the time required until the output from the downstream exhaust gas sensor becomes equal to the output from the upstream exhaust gas sensor, i.e., a parameter to evaluate a deteriorated state of the exhaust gas adsorbent.
Japanese laid-open patent publication No. 8-93458 discloses another process of evaluating a deteriorated state of the exhaust gas adsorbent with an HC sensor.
According to the discloses process, the HC sensor is disposed downstream of the exhaust gas adsorbent, and it is determined that the exhaust gas adsorbent is deteriorated if the HC concentration detected by the HC sensor is lower than a predetermined value in the absorption period and the desorption period, or alternatively it is determined that the exhaust gas adsorbent is deteriorated if the total amount of HC, produced when the product of the HC concentration detected by the HC sensor and the rate of the exhaust gas is integrated with respect to time in the desorption period is lower than a predetermined value in the desorption period.
However, because the HC sensor is also sensitive to exhaust gas components other than HC, it is generally difficult to properly detect the HC concentration at a location where the HC sensor is positioned, i.e., downstream of the exhaust gas adsorbent, from the output of the HC sensor. Consequently, it is difficult to obtain an appropriate evaluation of a deteriorated state of the exhaust gas adsorbent based on the HC concentration detected by the HC sensor.
It is therefore an object of the present invention to provide a method of accurately and appropriately evaluating a deteriorated state of an exhaust gas adsorbent which absorbs a particular component, e.g., HC, of an exhaust gas, specifically an exhaust gas adsorbent which operates alternatively in an adsorption mode for adsorbing a particular component of an exhaust gas and a desorption mode for desorbing the adsorbed component depending on an environmental condition such as temperature or the like.
According to the findings by the inventors of the present invention, exhaust gas sensors for generating respective outputs depending on the concentration of a particular component of an exhaust gas that is adsorbed by an exhaust gas adsorbent are disposed upstream and downstream, respectively, of the exhaust gas adsorbent in an exhaust passage which is supplied with the exhaust gas from an internal combustion engine, and the outputs generated by the exhaust gas sensors are affected by not only the concentration of the particular component in the exhaust gas at the exhaust gas sensors, but also the concentrations of other gas components. However, when the outputs generated by the exhaust gas sensors are sampled while the exhaust gas from the internal combustion engine is being supplied to the exhaust passage with the exhaust gas adsorbent disposed therein in an adsorption mode of the exhaust gas adsorbent, and the difference between the outputs generated by the exhaust gas sensors are observed, the difference is strongly correlated to the amount of the particular component, specifically the amount of the particular component per unit time, that is adsorbed by the exhaust gas adsorbent from instant to instant. The reason appears to be that basically, the concentration of only the particular component adsorbed by the exhaust gas adsorbent changes between a location upstream of the exhaust gas adsorbent and a location downstream of the exhaust gas adsorbent. Therefore, the amount of the particular component which has been adsorbed by the exhaust gas adsorbent per unit time can be determined on the basis of the difference between the outputs generated by the exhaust gas sensors while the exhaust gas from the internal combustion engine is being supplied to the exhaust passage and the particular component is being adsorbed by the exhaust gas sensors.
Since a deteriorated state of the exhaust gas adsorbent is a state in which the amount of the particular component that can be adsorbed by the exhaust gas adsorbent is smaller than the amount of the particular component that can be adsorbed when the exhaust gas adsorbent is brand new, the deterioration of the exhaust gas adsorbent is directly reflected in the adsorbed amount of the particular component thus determined.
Consequently, when the amount of the particular component adsorbed by the exhaust gas adsorbent is determined from the difference between the outputs generated by the exhaust gas sensors, a deteriorated state of the exhaust gas adsorbent can appropriately be evaluated.
According to an aspect of the present invention, there is provided a method of evaluating a deteriorated state of an exhaust gas adsorbent disposed in an exhaust passage supplied with an exhaust gas emitted from an internal combustion engine, said exhaust gas adsorbent being operable alternatively in an adsorption mode for adsorbing a particular component of the exhaust gas and a desorption mode for desorbing the adsorbed component depending on an environmental condition at the time said exhaust gas is supplied to the exhaust passage, comprising the steps of providing a first exhaust gas sensor and a second exhaust gas sensor for generating respective outputs depending on the concentration of said particular component, respectively upstream and downstream of said exhaust gas adsorbent, adsorbing said particular component with said exhaust gas adsorbent in said adsorption mode while supplying the exhaust gas from said internal combustion engine to said exhaust passage, calculating an amount of said particular component which has been adsorbed by said exhaust gas adsorbent per unit time, based on the difference between the outputs of said first and second exhaust gas sensors which is obtained while said particular component is being adsorbed by said exhaust gas adsorbent, and evaluating a deteriorated state of said exhaust gas adsorbent using the calculated adsorbed amount of said particular component.
In the above method, since an amount of said particular component which has been adsorbed by said exhaust gas adsorbent per unit time is calculated based on the difference between the outputs of said first and second exhaust gas sensors which is obtained while said particular component is being adsorbed by said exhaust gas adsorbent, the adsorbed amount of said particular component can accurately be determined without being essentially affected by exhaust gas components other than the particular component. Using the adsorbed amount of said particular component thus determined, a deteriorated state of said exhaust gas adsorbent can appropriately be evaluated.
The environmental condition mentioned above refers to a temperature of the exhaust gas adsorbent, for example. The particular component in the exhaust gas that is adsorbed by the exhaust gas adsorbent may be HC, NOx, or the like. If the particular component is HC, then the exhaust gas adsorbent comprises a zeolite-based adsorbent. The exhaust gas adsorbent of this type operates in an adsorption mode for adsorbing HC and a desorption mode for desorbing HC depending on the temperature of the exhaust gas adsorbent. If the particular component is NOx, then the exhaust gas adsorbent comprises an alkaline-metal-based adsorbent or alkaline-earch-based adsorbent. The exhaust gas adsorbent of this type operates in an adsorption mode for adsorbing HC and a desorption mode for desorbing NOx depending on the air-fuel ratio of the exhaust gas.
If the particular component is HC, then each of the first and second exhaust gas sensors for generating outputs depending on the concentration of HC may comprise an O2 sensor, an air-fuel ratio sensor, an HC sensor, or the like. The first and second exhaust gas sensors may be one of these sensor types. If the particular component is NOx, then each of the first and second exhaust gas sensors may comprise an O2 sensor, an air-fuel ratio sensor, an NOx sensor, or the like.
The particular component should preferably be adsorbed in a predetermined period immediately after the internal combustion engine has started to operate. Specifically, the particular component should be adsorbed from the start of operation of the internal combustion engine until the adsorbed amount of the particular component which has been calculated drops below a predetermined amount, i.e., becomes substantially nil, or until a predetermined time has elapsed from the start of operation of the internal combustion engine.
The step of calculating the adsorbed amount of said particular component preferably comprises the step of converting the difference between the outputs of said first and second exhaust gas sensors depending on a flow rate of the exhaust gas flowing in said exhaust passage when the outputs of said first and second exhaust gas sensors are obtained, and a temperature of said exhaust gas, for thereby determining the adsorbed amount of said particular component.
Specifically, inasmuch as the difference between the outputs of said first and second exhaust gas sensors corresponds to the concentration of the particular component that is adsorbed by the exhaust gas adsorbent, the amount of the particular component that is adsorbed by the exhaust gas adsorbent per unit time is affected by the flow rate of the exhaust gas at the time the outputs of said first and second exhaust gas sensors are acquired. The density of the particular component in the exhaust gas is affected by the temperature of the exhaust gas. In view of these considerations, the difference between the outputs of said first and second exhaust gas sensors is converted depending on the flow rate of the exhaust gas and the temperature of the exhaust gas thereby to determine the adsorbed amount of the particular component. Therefore, the adsorbed amount of the particular component can be determined with high accuracy.
The difference between the outputs of said first and second exhaust gas sensors may be converted by multiplying the difference by the flow rate of the exhaust gas supplied to the exhaust passage and a predetermined corrective coefficient depending on the temperature of the exhaust gas, using a predetermined data table.
According to the present invention, a deteriorated state of the exhaust gas adsorbent is evaluated, using the adsorbed amount of the particular component, as follows:
According to one process (hereinafter referred to as xe2x80x9cfirst evaluating processxe2x80x9d), the calculated adsorbed amount of said particular component is compared with a predetermined threshold to evaluate a deteriorated state of said exhaust gas adsorbent.
Specifically, when the exhaust gas adsorbent is deteriorated, the calculated adsorbed amount of said particular component, i.e., the adsorbed amount per unit time, becomes smaller than when the exhaust gas adsorbent is normal. Therefore, a deteriorated state of said exhaust gas adsorbent can be evaluated by comparing the adsorbed amount of said particular component with a predetermined threshold.
According to another process (hereinafter referred to as xe2x80x9csecond evaluating processxe2x80x9d), the adsorbed amount of said particular component is successively determined while said step of calculating an adsorbed amount of said particular component is being carried out, and said step of evaluating a deteriorated state of said exhaust gas adsorbent comprises the steps of measuring an elapsed time required from the start of said step of adsorbing said particular component until said successively determined adsorbed amount of said particular component drops to or below a predetermined amount, and comparing the measured elapsed time with a predetermined threshold thereby to evaluate a deteriorated state of said exhaust gas adsorbent.
Specifically, there is a certain limitation on the amount of the particular component that can be adsorbed by the exhaust gas adsorbent. When the exhaust gas adsorbent becomes nearly saturated as a result of adsorbing the particular component, the amount of the particular component that is adsorbed by the exhaust gas adsorbent per unit time becomes small. Furthermore, the total amount of the particular component that can be adsorbed by the exhaust gas adsorbent after the start of the step of adsorbing the particular component until the exhaust gas adsorbent becomes saturated is smaller when the exhaust gas adsorbent is deteriorated than when the exhaust gas adsorbent is normal. Consequently, if the elapsed time required from the start of said step of adsorbing said particular component until said successively determined adsorbed amount of said particular component drops to or below a predetermined amount, e.g., becomes substantially nil, as the exhaust gas adsorbent becomes nearly saturated, is measured, then the measured time becomes shorter when the exhaust gas adsorbent is deteriorated than when the exhaust gas adsorbent is normal. By comparing the measured time with a predetermined threshold, therefore, a deteriorated state of said exhaust gas adsorbent can appropriately be evaluated.
According to still another process (hereinafter referred to as xe2x80x9cthird evaluating processxe2x80x9d), the adsorbed amount of said particular component is successively determined while said step of calculating an adsorbed amount of said particular component is being carried out, and said step of evaluating a deteriorated state of said exhaust gas adsorbent comprises the steps of integrating said calculated adsorbed amount for a predetermined period while said step of adsorbing said particular component is being carried out, thereby to determine an integrated adsorbed amount of said particular component adsorbed by said exhaust gas adsorbent in said predetermined period, and comparing said integrated adsorbed amount with a predetermined threshold thereby to evaluate a deteriorated state of said exhaust gas adsorbent.
Specifically, as described above, the amount of the particular component that is adsorbed by the exhaust gas adsorbent per unit time becomes small when the exhaust gas adsorbent is deteriorated. When the adsorbed amount of the particular component is integrated for a predetermined period while said step of adsorbing said particular component is being carried out, thereby to determine an integrated adsorbed amount of said particular component, which represents the total amount of the particular component that is adsorbed by the exhaust gas adsorbent for the predetermined period, the integrated adsorbed amount of said particular component is smaller when the exhaust gas adsorbent is deteriorated than when the exhaust gas adsorbent is normal. By comparing the integrated adsorbed amount with a predetermined threshold, therefore, a deteriorated state of said exhaust gas adsorbent can appropriately be evaluated.
The predetermined period preferably comprises a period in which said adsorbed amount calculated from the start of said step of adsorbing said particular component drops to or below a predetermined amount, or a period in which a predetermined time elapses from the start of said step of adsorbing said particular component. The predetermined period may be the same as the period in which the step of adsorbing the particular component is carried out.
After the step of adsorbing said particular component has been carried out to cause the exhaust gas adsorbent to adsorb the particular component for a predetermined period, all the adsorbed particular component is desorbed from the exhaust gas adsorbent while the exhaust gas is being supplied to the exhaust passage in the desorption mode of the exhaust gas adsorbent. At this time, the total amount of the particular component desorbed from the exhaust gas adsorbent is basically considered to be equal to the total amount of the particular component that has been adsorbed by the exhaust gas adsorbent. While the particular component is being desorbed from the exhaust gas adsorbent, the difference between the outputs of said first and second exhaust gas sensors corresponds to the amount of the particular component desorbed from the exhaust gas adsorbent per unit time. Therefore, the amount of the particular component that has been desorbed by the exhaust gas adsorbent can be determined based on the difference between the outputs of said first and second exhaust gas sensors. By successively determining and integrating the amount of the particular component desorbed by the exhaust gas adsorbent, the total amount of the particular component desorbed from the exhaust gas adsorbent can be recognized. Stated otherwise, the total amount of the particular component that has been adsorbed by the exhaust gas adsorbent can indirectly be recognized from the integrated value of the desorbed amount of the particular component.
According to another aspect of the present invention, there is provided a method of evaluating a deteriorated state of an exhaust gas adsorbent disposed in an exhaust passage supplied with an exhaust gas emitted from an internal combustion engine, said exhaust gas adsorbent being operable alternatively in an adsorption mode for adsorbing a particular component of the exhaust gas and a desorption mode for desorbing the adsorbed component depending on an environmental condition at the time said exhaust gas is supplied to the exhaust passage, comprising the steps of providing a first exhaust gas sensor and a second exhaust gas sensor for generating respective outputs depending on the concentration of said particular component, respectively upstream and downstream of said exhaust gas adsorbent, adsorbing said particular component with said exhaust gas adsorbent in said adsorption mode while supplying the exhaust gas from said internal combustion engine to said exhaust passage for a predetermined period, thereafter, desorbing said particular component from said exhaust gas adsorbent in said desorption mode while supplying the exhaust gas from said internal combustion engine to said exhaust passage, successively calculating an amount of said particular component which has been desorbed from said exhaust gas adsorbent per unit time, based on the difference between the outputs of said first and second exhaust gas sensors which is obtained while said particular component is being desorbed from said exhaust gas adsorbent, integrating the desorbed amount of said particular component from the start of said step of desorbing said particular component from said exhaust gas adsorbent until the calculated desorbed amount of said particular component becomes substantially nil, thereby to determine an integrated desorbed amount of said particular component desorbed from said exhaust gas adsorbent in said step of desorbing said particular component, and comparing said integrated desorbed amount with a predetermined threshold thereby to evaluate a deteriorated state of said exhaust gas adsorbent.
In the above method, the integrated desorbed amount of said particular component is obtained by integrating the desorbed amount of said particular component from the start of said step of desorbing said particular component from said exhaust gas adsorbent until the calculated desorbed amount of said particular component becomes substantially nil, i.e., until the desorption of the particular component from the exhaust gas adsorbent is completed. Therefore, the integrated desorbed amount of said particular component represents the total amount of the particular component that has been adsorbed by the exhaust gas adsorbent in the period in which the step of adsorbing said particular component is carried out, and corresponds to the integrated adsorbed amount of said particular component in the third evaluating process. Because the desorbed amount of said particular component can accurately be determined based on the difference between the outputs of the first and second exhaust gas sensors, the integrated desorbed amount of said particular component can also be obtained with accuracy. As with the integrated adsorbed amount of said particular component in the third evaluating process, the integrated desorbed amount of said particular component becomes smaller when the exhaust gas adsorbent is deteriorated than when the exhaust gas adsorbent is normal. Thus, a deteriorated state of the exhaust gas adsorbent can appropriately be evaluated by comparing the integrated desorbed amount of said particular component with a predetermined threshold. The above process of evaluating a deteriorated state of the exhaust gas adsorbent based on the integrated desorbed amount of said particular component is referred to as xe2x80x9cfourth evaluating processxe2x80x9d.
In the fourth evaluating process, the step of calculating the desorbed amount of said particular preferably comprises the step of converting the difference between the outputs of said first and second exhaust gas sensors depending on a flow rate of the exhaust gas flowing in said exhaust passage when the outputs of said first and second exhaust gas sensors are obtained, and a temperature of said exhaust gas, e.g., multiplying the difference by a corrective coefficient determined depending on the flow rate of the exhaust gas and the temperature of the exhaust gas, for thereby determining the desorbed amount of said particular component. In this manner, the desorbed amount of said particular component can accurately be determined.
In the first through fourth evaluating processes, the predetermined threshold preferably is set depending on at least a temperature of said exhaust gas adsorbent.
Specifically, the amount of the particular component that is adsorbed by the exhaust gas adsorbent in the step of adsorbing the particular component generally varies depending on the temperature of the exhaust gas adsorbent. For example, a zeolite-based exhaust gas adsorbent for adsorbing HC tends to adsorb more HC as the temperature of the exhaust gas adsorbent is lower.
By setting the predetermined threshold depending on the temperature of said exhaust gas adsorbent, a deteriorated state of the exhaust gas adsorbent can more appropriately be evaluated.
The present invention is based on the assumption that the concentrations of exhaust gas components other than the particular component adsorbed by or desorbed from the exhaust gas adsorbent remain unchanged upstream and downstream of the exhaust gas adsorbent. However, the above assumption may not apply due to delay characteristics of the exhaust gas adsorbent and the effect of disturbance. If the above assumption is not applicable, then the difference between the outputs of the first and second exhaust gas sensors tends to contain an error, and hence the accuracy of the adsorbed amount and the desorbed amount that are determined based on the difference is lowered.
According to the present invention, a model which expresses the output per predetermined control cycle of said second exhaust gas sensor is constructed with a plurality of time-series data in a previous control cycle of the output of said first exhaust gas sensor, coefficient parameters relative to the time-series data, respectively, of said first exhaust gas sensor, and a monomial parameter independent of the time-series data of said first exhaust gas sensor. The method which determines the amount of the particular component adsorbed by the exhaust gas adsorbent per unit time further comprises the steps of successively identifying values of said coefficient parameters and said monomial parameter based on the outputs of said first and second exhaust gas sensors while said step of adsorbing said particular component is being carried out, and successively estimating the output of said second exhaust gas sensor using the identified values of said coefficient parameters and said monomial parameter based on said model, the step of calculating the adsorbed amount of said particular component comprising the step of determining said adsorbed amount using the difference between the output of said first exhaust gas sensor and the estimated output of said second exhaust gas sensor instead of the difference between the outputs of said first and second exhaust gas sensors.
The method which determines the amount of the particular component desorbed from the exhaust gas adsorbent per unit time further comprises the steps of successively identifying values of said coefficient parameters and said monomial parameter based on the outputs of said first and second exhaust gas sensors while said step of adsorbing said particular component is being carried out, and successively estimating the output of said second exhaust gas sensor using the identified values of said coefficient parameters and said monomial parameter based on said model, said step of calculating the desorbed amount of said particular component comprising the step of determining said desorbed amount using the difference between the output of said first exhaust gas sensor and the estimated output of said second exhaust gas sensor instead of the difference between the outputs of said first and second exhaust gas sensors.
As described above, while values of said coefficient parameters and said monomial parameter are being successively identified based on the outputs of said first and second exhaust gas sensors while said step of adsorbing or desorbing said particular component is being carried out, the output of said second exhaust gas sensor is successively estimated using the identified values of said coefficient parameters and said monomial parameter based on said model. According to the findings by the inventors, the difference between the output of said first exhaust gas sensor and the estimated output of said second exhaust gas sensor is free of an error that is contained in the difference between the outputs of said first and second exhaust gas sensors.
By using difference between the output of said first exhaust gas sensor and the estimated output of said second exhaust gas sensor instead of the difference between the outputs of said first and second exhaust gas sensors for calculating the adsorbed amount of the particular component and the desorbed amount of the particular component, the adsorbed amount of the particular component and the desorbed amount of the particular component can accurately be determined. Using the adsorbed amount of the particular component and the desorbed amount of the particular component thus determined, a deteriorated state of the exhaust gas adsorbent can more accurately be evaluated according to either one of the first through fourth evaluating processes.
If the outputs in each control cycle of the first and second exhaust gas sensors are represented by VS1 (k), VS2 (k) (k represents the ordinal number of a control cycle), then the above model is expressed by the following equation (1):
VS2 (k)=a1xc2x7VS1 (k)+ . . . +amxc2x7VS1 (kxe2x88x92m+1)+b1xe2x80x83xe2x80x83(1)
where m represents a natural number of 2 or more (e.g., m=4), a1, . . . , am represent the coefficient parameters, and b1 represents the monomial parameter.
Preferably, the step of identifying values of said coefficient parameters and said monomial parameter comprises the step of identifying values of said coefficient parameters and said monomial parameter according to an algorithm constructed to minimize an error between the value of the output (corresponding to the estimated output referred to above) of said second exhaust gas sensor which is determined from the time-series data of the output of said first exhaust gas sensor based on said model and the value of the actual output of said second exhaust gas sensor.
By thus identifying the values of the coefficient parameters and said monomial parameter according to the above algorithm, the above error corresponds to the error contained in the difference between the actual outputs of the first and second exhaust gas sensors. When the difference between the actual output of the first exhaust gas sensor and the estimated output of the second exhaust gas sensor based on the model is determined, the determined difference is free from an error contained in the difference between the actual outputs of said first and second exhaust gas sensors. As a result, the adsorbed amount of the particular component and the desorbed amount of the particular component can highly accurately be determined based on the difference between the actual output of the first exhaust gas sensor and the estimated output of the second exhaust gas sensor.
The above algorithm may comprise the algorithm of a method of least squares, a method of weighted least squares, a fixed gain method, a degression method, or the like.
The above and other objects, features, and advantages of the present invention will become apparent from the following description when taken in conjunction with the accompanying drawings which illustrate a preferred embodiment of the present invention by way of example.